A sterically demanding enolate ligand: Tantalum ligation and pyridine coupling

Enolate ligands of formula [OC(Ad)Ar](-) (Ad = 2-adamantylidene; Ar = 3,5-C6H3Me2 1a, Ar = 2,4,6-C6H2Me3 1b) were designed and prepared as potassium salts to mimic the topology of related N-tert-hydrocarbyl and ketimide ligands. Subsequently, trimethyltantalum bis-enolate complexes TaMe3(O-C[Ad]Ar)2 (Ar = 3,5-C6H3Me2 2a, Ar = 2,4,6-C6H2Me3 2b) were prepared in good yields from a salt metathesis reaction between TaMe3Cl2 and 2 equiv of the corresponding potassium enolate. Complex 2a was structurally characterized and found to be monomeric in the solid state and to exhibit an inner coordination sphere of approximate D-3h symmetry. Reaction between 2a and pyridine in the presence of dihydrogen resulted in an unanticipated pyridine coupling with formation of the bipyridine complex TaMe(O-C[Ad](3,5-C6H3Me2))(2)(py)(bpy), 3a (py = pyridyl, bpy = 2,2'-bipyridyl), which was structurally characterized to confirm its formulation. Labeling experiments using pyridine-d(5) and D-2 suggest that formation of the tantalum(III) complex 3a occurred via C-H activation involving a putative tantalum(III) intermediate, TaMe(O-C[Ad](3,5-C6H3Me2))(2)(py)(3), 4a.